How to fix Disqus width on page?

Hello ALL - I have installed Disqus on one page , but is taking ALL width on screen.this is what I have on the HTML code (at top on the Code Editor )

div id="disqus_thread">

and this on the CSS (at bottom)

BLOCKQUOTE { font-size: 18px; color: #424242;}

----------------------------------------------reading on some places say that I just need to change the SIZE of "disqus_thread" How can I do that?....I just want Disqus to be small and centered.... Thanks